Method and an electrowinning cell for production of metal

ABSTRACT

The present invention relates to a method for production of molten aluminium by electrolysis of an aluminous ore, preferably alumina, in a molten salt mixture, preferably a sodium fluoride—aluminium fluoride-based electrolyte. The invention describes an electrolysis cell for said production of aluminium by use of essentially inert electrodes in a vertical an/or inclined position, where said cell design facilitates separation of aluminium and evolved oxygen gas by providing a gas separation chamber ( 14 ) arranged in communication with the electrolysis chamber ( 22 ), thus establishing an electrolyte flow between the electrolysis chamber ( 22 ) and the gas separation chamber ( 14 ).

The present invention relates to a method and an electrowinning cell forthe production of aluminium, in particular electrowinning of aluminiumby the use of substantially inert electrodes.

Aluminium is presently produced by electrolysis of analuminium-containing compound dissolved in a molten electrolyte, and theelectrowinning process is performed in cells of conventionalHall-Hèroult design. These electrolysis cells are equipped withhorizontally aligned electrodes, where the electrically conductiveanodes and cathodes of today's cells are made from carbon materials. Theelectrolyte is based on a mixture of sodium fluoride and aluminiumfluoride, with smaller additions of alkaline and alkaline earthfluorides. The electrowinning process takes place as the current passedthrough the electrolyte from the anode to the cathode causes theelectrical discharge of aluminium-containing ions at the cathode,producing molten aluminium, and the formation of carbon dioxide at theanode (see Haupin and Kvande, 2000). The overall reaction of the processcan be illustrated by the equation:2Al₂O₃+3C=4Al+3CO₂   (1)

Due to the horizontal electrode configuration, the preferred electrolytecomposition and the use of consumable carbon anodes, the currently usedHall-Hèroult process displays several shortcomings and weaknesses. Thehorizontal electrode configuration renders necessary an area-intensivedesign of the cell, which results in a low aluminium production raterelative to the footprint of the cell. The low productivity to arearatio causes high investment cost for greenfield primary aluminiumplants.

The traditional aluminium production cells utilise carbon materials asthe electrically conductive cathode. Since carbon is not wetted bymolten aluminium, it is necessary to maintain a deep pool of moltenaluminium metal above the carbon cathode, and it is in fact the surfaceof the aluminium pool that is the “true” cathode in the present cells. Amajor drawback of this metal pool is that the high amperage of moderncells (>150 kA) creates considerable magnetic forces, disturbing theflow patterns of the electrolyte and the metal in the electrowinningcells. As a result, the metal tends to move around in the cell causingwave movements that might locally shortcut the cell and promotedissolution of the produced aluminium into the electrolyte. In order toovercome this problem, complex busbar systems are designed to compensatefor the magnetic forces and to keep the metal pool as stable and flat aspossible. The complex busbar system is costly, and if the disturbance ofthe metal pool is too large, aluminium dissolution in the electrolytewill be enhanced, resulting in reduced current efficiency due to theback reaction:2Al+3CO₂=Al₂O₃+3CO   (2)

The preferred carbon anodes of today's cells are consumed in the processaccording to reaction (1), with a typical gross anode consumption of 500to 550 kg of carbon per tonne of aluminium produced. The use of carbonanodes results in the production of pollutant greenhouse gases like CO₂and CO in addition to the so-called PFC gases (CF₄, C₂F₆, etc.). Theconsumption of the anode in the process means that the interpolardistance in the cell will constantly change, and the position of theanodes must be frequently adjusted to keep the optimum operatinginterpolar distance. Additionally, each anode is replaced with a newanode at regular intervals. Even though the carbon material and themanufacture of the anodes are relatively inexpensive, the handling ofthe used anodes (butts) makes up a major portion of the operating costin a modern primary aluminium smelter.

The raw material used in the Hall-Hèroult cells is aluminium oxide, alsocalled alumina. Alumina has a relatively low solubility in mostelectrolytes. In order to achieve sufficient alumina solubility, thetemperature of the molten electrolyte in the electrowinning cell must bekept high. Today, normal operating temperatures for Hall-Hèroult cellsare in the range 940–970° C. To maintain the high operatingtemperatures, a considerable amount of heat must be generated in thecell, and the major portion of the heat generation takes place in theinterpolar space between the electrodes. Due to the high electrolytetemperature, the side walls of today's aluminium production cells arenot resistant to the combination of oxidising gases and cryolite-basedmelts, so the cell side linings must be protected during cell operation.This is normally achieved by the formation of a crust of frozen bathledge on the side walls. The maintenance of this ledge necessitatesoperating conditions where high heat losses through the side walls is anecessary requirement. This results in the electrolytic productionhaving an energy consumption that is substantially higher that thetheoretical minimum for aluminium production. The high resistance of thebath in the interpolar space accounts for 35–45% of the voltage lossesin the cell. The state-of-the-art of present technology is cellsoperating at current loads in the range 250–350 kA, with energyconsumption around 13 kWh/kg Al and a current efficiency of 94–95%.

The carbon cathodes used in the traditional Hall-Hèroult cells arevulnerable to sodium swelling and erosion, and both of these can causecell life reduction.

As pointed out, there are several good reasons for improving the celldesign and the electrode materials in aluminium electrolysis cells, andseveral attempts have been made to obtain these improvements. Onepossible solution to overcome some of the problems experienced in thepresently used Hall-Hèroult cells, is the introduction of so-calledwettable (or inert) cathodes. The introduction of aluminium wettablecathodes has been suggested in several patents, among others U.S. Pat.Nos. 3,400,036, 3,930,967 and 5,667,664. All of the patents in thisfield of invention are aimed at reducing the energy consumption duringaluminium electrolysis through the implementation of so-called aluminiumwettable cathode materials. The energy reduction during electrolysis isaccomplished by the construction of an electrolytic cell with drainedcathodes, allowing for cell operation without the presence of analuminium pool. Most of the patents are related to the retrofit of theconventional Hall-Hèroult cell types, although some presuppose theintroduction of novel cell designs. Wettable cathodes are proposedmanufactured from so-called Refractory Hard Materials (RHM) likeborides, nitrides and carbides of the transition metals, and also RHMsilicides are proposed as useful inert cathodes. The RHM cathodes arereadily wetted by aluminium and hence a thin film of aluminium may bemaintained on the cathode surfaces during aluminium electrowinning indrained cathode configurations. Due to the high cost of the RHMmaterials, the manufacture of RHM/graphite composites, for instanceTiB₂—C composite, constitutes a viable alternative material for drainedcathodes. The wettable cathodes can be inserted in the proposedelectrolysis cells as solid cathode structures or as slabs, “mushrooms”,lumps, plates, etc. The materials may also be applied as surface layersas slurries, pastes, etc., that adhere to the underlying substrate,usually carbon based, during start-up or preheating of the cell orcathode elements (for instance U.S. Pat. Nos. 4,376,690, 4,532,017 and5,129,998). As proposed in these patents, the RHM cathodes may beinserted as “pre-cathodes” that partially floats on top of theunderlying aluminium pool in the electrowinning cell, and as suchdecreases the interpolar distance and will also have a dampening effecton the metal movement in the cell bottom. Problems expected to beencountered during the operation of such “pre-cathode” cells are relatedto breaking of the shapes, stability of the mounted elements andlong-time operational stability. Brown et al. (1998) have reportedsuccessful operation for a relatively short time period of Hall-Hèroultcells using TiB₂/C-composite wettable cathodes in a drainedconfiguration, but as known to those skilled in the art, long-timeoperation will be problematic due to the dissolution of TiB₂ resultingin removal of the wettable cathode layer on top of the carbon cathodeblocks. The introduction of wettable cathodes and so-called“pre-cathodes” in Hall-Hèroult cells with their horizontal electrodealignment, however, do not address the low area utilisation of saidcells.

With an inert anode in the electrowinning of aluminium, the overallreaction would be:2Al₂O₃=2Al+3O₂   (3)

So far, no commercial-scale electrolysis cells have been operatedsuccessfully over longer periods of time with inert anodes. Manyattempts have been made to find the optimum inert anode material and theintroduction of these materials in electrolytic cells, and numerouspatents have been proposed for inert anode materials for aluminiumelectrowinning. Most of the proposed inert anode materials have beenbased on tin oxide and nickel ferrites, where the anodes may be a pureoxide material or a cermet type material. The first work on inert anodeswas initiated by C. M. Hall, who worked with copper metal (Cu) as apossible anode material in his electrolysis cells. Generally, the inertanodes can be divided into metal anodes, oxide-based ceramic anodes andcermets based on a combination of metals and oxide ceramics. Theproposed oxide-containing inert anodes may be based on one or more metaloxides, wherein the oxides may have different functions, as for instancechemical “inertness” towards cryolite-based melts and high electricalconductivity. The proposed differential behaviour of the oxides in theharsh environment of the electrolysis cell is, however, questionable.The metal phase in the cermet anodes may likewise be a single metal or acombination of several metals (metal alloys). The main problem with allof the suggested anode materials is their chemical resistance to thehighly corrosive environment due to the evolution of pure oxygen gas (1bar) and the cryolite-based electrolyte. To reduce the problems of anodedissolution into the electrolyte, additions of anode material components(U.S. Pat. No. 4,504,369) and a self generating/repairing mixture ofcerium based oxyfluoride compounds (U.S. Pat. Nos. 4,614,569, 4,680,049and 4,683,037) have been suggested as possible inhibitors of theelectrochemical corrosion of the inert anodes. However, none of thesesystems have been demonstrated as viable solutions.

When operating cells with inert anodes, one problem often run into isthe accumulation of anode material elements in the aluminium produced.Several patents have tried to address these problems by suggesting areduction in the cathode surface area, i.e. the surface of the aluminiumproduced. Reduced aluminium surface area exposed to the electrolyticbath will reduce the uptake of dissolved anode material components inthe metal, and hence increase the durability of the oxide-ceramic (ormetal or cermet) anodes in the electrolysis cells. This is amongstothers described in U.S. Pat. Nos. 4,392,925, 4,396,481, 4,450,061,5,203,971, 5,279,715 and 5,938,914 and in GB 2 076 021.

Other publications related to this technical field are as follows:

-   -   Haupin, W. and Kvande, H.: “Thermodynamics of electrochemical        reduction of alumina”, Light Metals 2000, 379–384.    -   Pawlek, R. P.: “Aluminium wettable cathodes: An update”, Light        Metals 1998, 449–454.    -   Brown, G. D., Hardie, G. J., Shaw, R. W. and Taylor, M. P.: “TiB        ₂ coated aluminium reduction cells: Status and future direction        of coated cells in Comalco”, Proceedings of the 6^(th)        Australasian Al Smelting Workshop, Queenstown, New Zealand,        Nov.26, 1998.

The introduction of inert anodes and wettable cathodes in the presentHall-Hèroult electrowinning cells would have a significant impact onreducing the production of greenhouse gases like CO₂, CO and PFC's fromaluminium production. Also, potentially the reduction in energy addedwould be substantial if a drained cathode design could be employed.However, in order to really make substantial progress in theoptimisation of electrolytic aluminium production, both inert(dimensionally stable) anodes and wettable cathodes must be incorporatedin a novel cell design. Novel cell designs can be divided into twogroups, designs aimed at retrofit of existing Hall-Hèroult type cells,and completely new cell designs.

Patents regarding retrofit or enhanced development of Hall-Hèroult cellsare amongst others described in U.S. Pat. Nos. 4,504,366, 4,596,637,4,614,569, 4,737,247, 5,019,225, 5,279,715, 5,286,359 and 5,415,742, aswell as GB 2 076 021. All of these patents address the problemsencountered due to the high heat losses in the present Hall-Hèroultcells, and the electrolysis process is operated at reduced interpolardistances. Some of the proposed designs are in addition effective withrespect to reducing the surface area of the liquid aluminium metal padexposed to the electrolyte. However, only a few of the suggested designshave addressed the low production to area ratio of the Hall-Hèroultcells. Amongst others, U.S. Pat. Nos. 4,504,366, 5,279,715 and 5,415,742have tried to solve this problem by implementation of vertical electrodeconfigurations to increase the total electrode area of the cell. Thesethree patents have also suggested the use of bipolar electrodes. Themajor problem of the cell design suggested in these patents, however, isthat the requirement for a large aluminium pool on the cell bottom toprovide electrical contact for the cathodes. This will render the cellsusceptible to the influence of the magnetic fields created by thebusbar system, and may hence cause local short-circuiting of theelectrodes.

U.S. Pat. Nos. 4,681,671, 5,006,209, 5,725,744 and 5,938,914 describenovel cell designs for aluminium electrowinning. Also U.S. Pat. Nos.3,666,654, 4,179,345, 5,015,343, 5,660,710 and 5,953,394, and Norwegianpatent no. NO 134495 describe possible designs of light metalelectrolysis cells, although one or more of these patents are orientedtowards magnesium production. Most of these cell concepts arc applicableto multi-monopolar and bipolar electrodes. The common denominator of allof the above suggested cells designs is a vertical electrodeconfiguration for the utilisation of the so-called gas lift effect. Asgas is evolved at the anode it raises towards the surface of theelectrolyte, creating a drag force that can be utilised to “pump” theelectrolyte in the cell. By suitable arrangement of the anodes andcathodes, this gas-lift induced flow of electrolyte can be controlled.All of these prior patents claim better current efficiencies, purermetal quality and improved metal—gas separation properties. However, forthe purpose of separating a produced metal that is denser than theelectrolyte, one general impression of the prior patents, as forinstance expressed in U.S. Pat. No. 5,660,710, is that the separation orpartition wall does not extend deep enough in the electrolyte toaccomplish this task. Additionally, several of the patents, for exampleNorwegian pat. No. 134495, introduce the term gas separation chambermerely by increasing the height of the free volume between theelectrolyte level above the electrodes and the lid of the electrolyticcell. This design change is, however, not sufficient to assure theremoval of finely dispersed oxygen bubbles in the electrolyte due to thehigh velocities of the electrolyte in the areas directly above andadjacent to the oxygen-evolving anodes in the cell.

Additionally, the referred patents, as well as U.S. Pat. No. 6,030,518,all point to the lowering of the bath temperature as compared to normalHall-Hèroult cell temperatures as a means of a feasible reduction of theanode corrosion rates in the cell. The utilisation of the gas-lifteffect and design of so-called up-comer and down-comer flow funnels arealso described in U.S. Pat. No. 4,308,116, specially aimed at magnesiumproduction.

U.S. Pat. No. 4,681,671 describes a novel cell design with a horizontalcathode and several, blade-shaped vertical anodes, and the cell is thenoperated at low electrolyte temperatures and with an anodic currentdensity at or below a critical threshold value at which oxide-containinganions are discharged preferentially to fluoride anions. By means offorced or natural convection, the melt is circulated to a separatechamber or a separate unit, in which alumina is added before the melt iscirculated back into the electrolysis compartment. Although the totalsurface area of the anode is high in the proposed configuration, theeffective anode area is small and limited due to the low electricalconductivity of the anode material relative to the electrolyte. Thiswill substantially limit the useful anodic surface area, and will leadto high corrosion rates at the effective anode surface.

The proposed cell design presented in U.S. Pat. No. 5,938,914 consistsof inert anodes and wettable cathodes in a completely closed arrangementfor ledge-free aluminium electrowinning. The cell is preferablyconstructed with a plurality of interleaved, vertical anodes andcathodes with an anode to cathode surface area ratio of 0.5–1.3. Thebath temperature is in the range from 700° C. to 940° C., with 900–920°C. as the preferred operating range. The electrode assembly has outerwalls that define a down-comer and an up-comer for the electrolyte flowinduced by the gas-lift effect of the oxygen bubbles produced at theanode(s). A roof is placed above the anodes to collect the gas and todirect the evolved oxygen into the up-comer defined in the electrolysischamber. The end cathodes are electrically connected to the cathode leadof the electrode assembly, whereas any interleaved cathode plates areelectrically connected to the end cathode plates by means of thealuminium pool on the cell floor.

An aluminium electrowinning cell with vertical electrodes and a metalcollection “sump” created by a drained cell floor design was proposed inU.S. Pat. No. 5,006,209. The electrolysis cell concept was designed formetal-based anodes and wettable cathodes where the electrolysis processtakes place in a fluoride-containing electrolyte at low temperatures,and where the aluminium ore is solid and dissolved alumina is kept insuspension in the electrolyte. Again, the convection pattern of theelectrolyte in the electrolysis cell is created by the so-calledgas-lift effect due to oxygen-evolving anodes. The cell floor itself isan auxiliary non-consumable anode, or the anodes may have an invertedT-shape, and is as such an oxygen-evolving “bottom” anode. A possibleproblem of this design is that aluminium produced on the cathodes andflowing downward will be exposed to the oxygen gas produced at the“bottom” anode and hence contribute to reduced current efficiencythrough the back reaction. Additionally, if aluminium comes into contactwith the oxide layer on the metal anode, an exothermic reaction betweenaluminium and the oxidised anodic layer will take place. This willcontribute to loss of current efficiency in the cell as well as to thedeterioration of the anode with subsequent contamination of the producedmetal. Another problem that is expected to be encountered duringlong-time operation of the cell described in U.S. Pat. No. 5,006,209, isthe accumulation of alumina-containing sludge in the cell bottom. Thisproblem is expected due to the low solubility of alumina at thesuggested operating temperatures, and the problems of keeping aluminafreely suspended in the cell during varying cell operating conditions(i.e. temperature fluctuations, bath composition fluctuations andalumina quality fluctuations).

U.S. Pat. No. 5,725,744 proposes a different concept for a novel designof an aluminium electrowinning cell. The cell is designed for preferredoperation at low temperatures, and thus requiring operation at lowanodic current densities. The inert electrodes and wettable cathodes arealigned vertically, or practically vertically, in the cell, thusmaintaining an acceptable cell footprint. The electrodes are aligned asseveral interleaved rows adjacent to the side walls of the cell oralternatively a single row of multi-monopolar electrodes along itslength. The anode surface area, and possibly the cathode area, areincreased by the use of a porous or reticulated skeletal structure,where the anode leads are introduced from the top of the cell and thecathode leads are introduced from the bottom or lower side walls. Thecell operates with an aluminium pool on the cell floor. Spacers are usedbetween or adjacent to the electrodes to maintain a fixed interpolardistance, and to provide the desired electrolyte flow pattern in thecell, i.e. an upward movement of the electrolyte flow in the interpolarspacing. The cell is likewise designed with a cell housing outside theelectrodes that provides a downward movement of the electrolyte. Aluminais fed into the cell in the cell housing with the downward electrolyteflow. According to the present authors' understanding, one of the mainproblems encountered with the proposed cell design of the said U.S.patent is the short-comings with respect to separation of the producedmetal and electrolyte. A large aluminium pool is prescribed to bepresent at the cell floor level, thus as in other similar electrowinningcell designs a large surface area of molten aluminium is in contact withthe electrolyte, enhancing the accumulation of dissolved anode materialin the produced metal, and enhancing the dissolution of aluminium in theelectrolyte. The latter problem will reduce the current efficiency ofthe cell through the back reaction with dissolved oxidising gas species,and the first will lead to reduced metal quality.

A fact well established in hydrodynamics is that the flow of a fluidsystem is governed by a balance between the driving force for fluid flowand the resistance to fluid flow within the components of the system.Furthermore, depending upon the configuration, the velocity within localregions flow may be in the same direction but may sometimes be in thedirection opposite to the fluid drive. This principle is amongst otherscited in U.S. Pat. Nos. 3,755,099, 4,151,061 and 4,308,116. Inclinedelectrode surfaces are used to enhance/facilitate the drainage of gasbubbles from the anode and molten metal from the cathode. Hence, thedesign of electrolysis cells with vertical or near horizontal electrodesof both multi-monopolar and bipolar electrode arrangement, where fixedinterpolar distance and the gas-lift effect are used to create a forcedconvection of the electrolyte flow, is not new. U.S. Pat. Nos.3,666,654, 3,779,699, 4,151,061 and 4,308,116, amongst others utilisesuch design principles, and the two latter patents also givedescriptions of the use of “funnels” for up-comer(s) and down-comer(s)with respect to the electrolyte flow. U.S. Pat. No. 4,308,116 alsosuggests the use of a separation wall for enhanced separation ofproduced metal and gas.

It is an object of the present invention to provide a method and anelectrowinning cell for production of aluminium by the electrowinning ofaluminous ore, preferably aluminium oxide, in a molten fluorideelectrolyte, preferably based on cryolite, at temperatures in the range680–980° C. The said method is designed to overcome problems related tothe present production technology for electrowinning of aluminium, andthus providing a commercial and economically viable process for saidproduction. This means the design of an electrolysis cell with thenecessary cell components and outline to reduce energy consumption,reduce overall production costs and still maintain high currentefficiency. The compact cell design is obtained by the use ofdimensionally stable anodes and aluminium wettable cathodes. Theinternal electrolyte flux is designed to attain a high dissolution rateof alumina, even at low electrolyte temperatures, and a good separationof the two products from the electrolysis process. Problems identifiedwith the mentioned patents (U.S. Pat. Nos. 4,681,671, 5,006,209,5,725,744 and 5,938,914) are also not encountered in this invention dueto the more sophisticated design of the electrolysis cell.

A governing principle in the present invention related to anelectrolysis cell for the accomplishment of aluminium electrolysis, andfor the construction principle of the aluminium electrowinning cell, isthat the two products, aluminium and oxygen, shall be efficientlycollected with minimal losses due to the recombination of theseproducts. The impediment of this recombination is accomplished throughrapid and complete separation of aluminium and oxygen. This is soughtrealised through the forced convection of the metal and thegas/electrolyte in opposite directions, in such a manner as to achievemaximal differences in the actual velocity vectors of the two products.

These and other advantages can be achieved by the invention as definedin the accompanying claims.

In the following, the invention shall be further described by figuresand an example where:

FIG. 1: Shows a schematic view of the vertical cross sectionlongitudinally of the electrolysis compartment of an electrolysis cellaccording to the invention,

FIG. 2: Shows a vertical cross section transverse of the electrolysiscell shown in FIG. 1.

FIGS. 1 and 2 disclose a cell for the electrowinning of aluminiumcomprising anodes 1 and cathodes 2 immersed in an electrolyte Econtained in an electrolysis chamber 22. In operation, the electrolytewill be separated from the upward rising gas bubbles 15 (FIG. 2) bydeflection in a direction more or less perpendicular to the gas streamin the interpolar space 18 (FIG. 1) between the interleavedmulti-monopolar or bipolar electrodes, where the gas is evolved at theinert anode surface 1. The electrolyte, containing some oxygen bubblesof smaller size (15) will be deflected into a gas separation chamber 14(FIG. 2) through one or more openings 12 in the partition wall 9. Inthis chamber the electrolyte flow rate is reduced to enhance the gasseparation. The gas-free electrolyte is then lead into the electrolysischamber through corresponding openings 13 in the partition wall,providing a flow of “fresh” electrolyte into the interpolar space 18. Inprincipal, the separation wall 9 can be constructed without openings(12, 13), and the circulation of the electrolyte between theelectrolysis chamber 22 and the gas separation chamber 14 can then beobtained by limiting the extent of the partition wall. In practice thiscan be achieved by allowing a gap between an auxiliary floor 10 and thelower end of the partition wall 9, and a gap of similar dimensionsbetween the top of the partition wall 9 and the upper electrolyte level.

The produced aluminium will flow downward on the aluminium wettablecathode surfaces 2 in the opposite direction of the electrolyte and therising gas bubbles. The produced aluminium will pass through holes 17 ofthe auxiliary cell floor 10, and will be collected in an aluminium pool11 shielded from the flowing electrolyte in a metal compartment 23. Themetal can be extracted from the cell through a hole suitably locatedthrough the cell lid 8, or through one or more surge pipes/siphons 19attached to the cell. It is a principle of the present invention toarrange the electrodes 1, 2 and the partition wall 9, as well as theauxiliary cell floor 10, so as to achieve a balance between thebuoyancy-generated bubble forces (gas-lift effect) on one side and theflow resistance on the other hand to give a net motion of theelectrolyte to provide the required alumina dissolution and supply, aswell as separation of the products. Preferably the partition wall 9extends between two opposing side walls 24, 25 of the cell. Its heightmay extend from the bottom 26 or the auxiliary floor of the cell andupward to at least the surface of the electrolyte. The height can belimited to allow full exchange of gas between the electrolysis chamber22 and the gas separating chamber 14.

The cell is located in a steel container 7, or in a container made ofanother suitable material. The container has a thermal insulating lining6 and a refractory lining 5 with excellent resistance to chemicalcorrosion by both fluoride-based electrolyte and produced aluminium 11.The floor of the cell is formed to create a natural drainage of thealuminium to a deeper pool for easy extraction of produced metal fromthe cell. Alumina is preferably fed through one or more pipes 20 andinto the highly turbulent flow region of the electrolyte in theelectrolysis chamber between the electrodes of the cell. This will allowa fast and reliable dissolution of alumina, even at low bathtemperatures and/or high cryolite ratios of the electrolyte. Optionally,the alumina can be fed into the gas separation chamber 14. Theelectrodes are connected to a peripheral busbar system throughconnections 3, in which the temperatures can be controlled through acooling system 4.

The off-gasses formed in the cell during the electrolysis process willbe collected in the top part of the cell above the gas separation andthe electrolysis chamber. The off-gases can then be extracted from thecell through an exhaust system 16. The exhaust system can be coupled tothe alumina feeding system 20 of the cell, and the hot off-gasses can beutilised for preheating of the alumina feed stock. Optionally, thefinely dispersed alumina particles in the feed stock may act as a gascleaning system, in which the off-gasses are completely and/or partiallystripped from any electrolyte droplets, particles, dust and/or fluoridepollutants in the off-gasses from the cell. The cleaned exhaust gas fromthe cell is then connected to the gas collector system (28) of thepotline.

The present cell design achieves reduced contact time and reducedcontact area between the metal and the electrolyte. Hence, theunfortunate consequence of previously known design solutions is avoided,where a relatively large surface area of molten aluminium is kept incontact with the electrolyte, and renders possible the enhancedaccumulation of dissolved anode material in the produced metal. Thecontact area of the cathode, i.e. the downward flowing aluminium may beeven further reduced by reducing the cathode surface area relative tothe anode surface area. A reduction in the exposed cathodic surface areawill reduce the contamination levels of anode material in the producedmetal, thus reducing the anodic corrosion during the electrolysisprocess. A reduction in the anodic corrosion can also be obtained byreducing the anodic current density and by lowering of the operatingtemperature.

A novel concept of the invented cell is the implementation of anauxiliary cell floor. By means of the gas produced at the anode, agas-lifting effect is created, setting up a desired circulation patternin the electrolyte. This circulation pattern transports the produced gasupward and away from the downward flowing aluminium. The optionalintroduction of diaphragms, interior walls or “skirts” 21 (FIG. 1)between the anodes 1 and the cathodes 2 may under certain circumstancesenhance the preferred circulation pattern of the electrolyte, and thediaphragms may also reduce the downward circulation of the electrolytealong the cathode surfaces by means of reducing the natural tendency fora downward movement of the electrolyte. Due to the large volume of thegas separation chamber 14 relative to the total interpolar volumes, thegas separation chamber will act as a de-gaser for any oxygen gas“trapped” in the electrolyte, thus allowing for an essentially gas-freeelectrolyte to be circulated back to the electrolysis chamber. Thecommunication between the electrolysis chamber and the gas separationchamber takes place through “openings” in the partition wall inserted inthe cell, and the size and position of these “openings” (12 and 13)determine the flow pattern as well as the flow rates in the cell.

The shown multi-monopolar anodes 1 and cathodes 2 may obviously bemanufactured as several smaller units and assembled to form an anode orcathode of the desired dimensions. In addition, except for the endelectrodes, all interleaved inert anodes 1 and aluminium wettablecathodes 2 can be exchanged by bipolar electrodes, which may be designedand positioned in the same manner. This alignment will cause the endelectrodes in the cell to act as a terminal anode and terminal cathode,respectively. The electrodes are preferably arranged in a verticalalignment, but cantilevered/tilted electrodes can also be used. Alsotracks (grooves) in the electrodes may be applied to improve theseparation and collection/accumulation of produced gas and/or metal.

Continuous operation of the electrolysis cell requires the use ofdimensionally stable inert anodes 1. The anodes are preferably made ofmetals, metal alloys, ceramic materials, oxide based cermets, oxideceramics, metal ceramic composites (cermets) or combinations thereof,with high electrical conductivity. The cathodes 2 must also bedimensionally stable and wettable by aluminium in order to operate thecell at constant interpolar distances 18, and the cathodes arepreferably made from titanium diboride, zirconium diboride or mixturesthereof, but may also be made from other electrically conductingrefractory hard metals (RHM) based on borides, carbides, nitrides orsilicides, or combinations and/or composites thereof. The electricalconnections to the anodes are preferably inserted through the lid 8 asshown in FIGS. 1 and 2. The connections to the cathodes may be insertedthrough the lid 8, through the long side walls 27 (FIG. 2) or throughthe cell bottom 26.

The invented cell can be operated at low interpolar distances 18 to saveenergy during aluminium electrowinning. The productivity of the cell ishigh, as vertical electrodes provide large electrode surface areas and asmall “footprint” of the cell. Low interpolar distances mean that theheat generated in the electrolyte is reduced compared to traditionalHall-Hèroult cells. The energy balance of the cell can hence beregulated by designing a correct thermal insulation 6 in the sides 24,25, 27 and the bottom 27 is necessary, as well as in the cell lid 8. Thecell can then optionally be operated without a frozen ledge covering theside walls, and chemically resistant cell materials is in such cases amatter of necessity. However, the cell can also be operated with afrozen ledge covering, at least parts of, the sidewalls 24, 25, 27 andbottom 26 of the cell.

Excess heat generated must be withdrawn from the cell through thewater-cooled electrode connections 3,4 and/or the use of auxiliary meansof cooling like heat pipes, etc. Depending on the desired heat balanceand operating conditions of the cell, the heat retracted from theelectrodes may be used for heat/energy recovery. The cell liner 5 ispreferably made of densely sintered refractory materials with excellentcorrosion resistance toward the used electrolyte and aluminium.Suggested materials are alumina, silicon carbide, silicon nitride,aluminium nitride, and combinations thereof or composites thereof.Additionally, at least parts of the cell lining can be protected fromoxidising or reducing conditions by utilising protective layers ofmaterials that differs from the bulk of the dense cell liner describedabove. Such protective layers can be made of oxide materials, forinstance aluminium oxide or materials consisting of a compound of one orseveral of the oxide components of the anode material and additionallyone or more oxide components. The auxiliary cell floor 10, partitionwall 9 and diaphragms 21 can also be made of densely sintered refractorymaterials with excellent corrosion resistance toward the usedelectrolyte and aluminium. Suggested materials are alumina, siliconcarbide, silicon nitride, aluminium nitride, and combinations thereof orcomposites thereof. The two latter units (9,21) can also utilise otherprotective materials in at least parts of the construction, where theprotective layers can be made of oxide materials, for instance aluminiumoxide or materials consisting of a compound of one or several of theoxide components of the anode material and additionally one or moreoxide components.

The shape and design of the degassing or gas separation chamber may varydepending on the production capacity of the cell. The gas separationchamber may in reality consist of several chambers placed on either sideof the electrolysis chamber, or consist of one or more chambersseparating two adjacent electrolysis compartments, or consist of one ormore chambers alongside the electrolysis chamber as shown in FIG. 2. Thegas separation chamber may also be opened during cell operation fordrainage/removal of any alumina sludge accumulated in the cell.

The invented cell is designed for operation at temperatures ranging from680° C. to 970° C., and preferably in the range 750–940° C. The lowelectrolyte temperatures are attainable by use of an electrolyte basedon sodium fluoride and aluminium fluoride, possibly in combination withalkaline and alkaline earth halides. The composition of the electrolyteis chosen to yield (relatively) high alumina solubility, low liquidustemperature and a suitable density to enhance the separation of gas,metal and electrolyte. In one embodiment, the electrolyte comprises amixture of sodium fluoride and aluminium fluoride, with possibleadditional metal fluorides of the group 1 and 2 elements in the periodictable according to the IUPAC system, and the possible components basedon alkali or alkaline earth halides up to a fluoride/halide molar ratioof 2.5, and where the NaF/AlF₃ molar ratio is in the range 1 to 3,preferably in the range 1.2–2.8.

It should be understood that the suggested aluminium electrowinning cellas presented in the example relating to FIGS. 1 and 2, represents onlyone particular embodiment of the cell, which may be used to perform themethod of electrolysis according to the invention.

1. A cell for electrolytic production of aluminium comprising at leastone electrolysis chamber containing an electrolyte, at least one inertanode and at least one wettable cathode, wherein: a gas separatingchamber is arranged in communication with said electrolysis chamber,where gas evolved in the electrolysis process is directed to flow intothe gas separation chamber thus establishing an electrolyte flow patternbetween the electrolysis chamber and the separation chamber, where gasevolved in the process can be separated from the electrolyte in the gasseparation chamber; the electrolysis chamber comprises an auxiliaryfloor; and at least one diaphragm, interior wall or skirt is positionedbetween at least one anode and at least one cathode.
 2. An electrolysiscell in accordance with claim 1, wherein: a partitioning wall isarranged between the electrolysis chamber and the gas separatingchamber, said wall having at least one opening formed therethrough. 3.An electrolysis cell in accordance with claim 2, wherein thepartitioning wall has at least one upper opening allowing thegas-containing electrolyte to flow from the electrolysis chamber to thegas separating chamber, and at least one lower opening through whichelectrolyte separated from the gas returns to the electrolysis chamber.4. An electrowinning cell in accordance with claim 2, wherein thepartitioning wall is manufactured from aluminum oxide, aluminiumnitride, silicon carbide, silicon nitride or combinations or compositesthereof.
 5. An electrowinning cell in accordance with claim 2, whereinthe partitioning wall is manufactured from oxide materials.
 6. Anelectrowinning cell in accordance with claim 2, wherein the partitioningwall is manufactured from oxide or materials consisting of a compound ofone or several of the oxide components of the anode material, andadditionally one or more oxide components.
 7. An electrowinning cell inaccordance with claim 2, wherein the partitioning wall extends betweentwo opposing side walls of the cell, where its height may extend fromthe bottom or the auxiliary floor of the cell and upward to at least theupper level of the electrolyte.
 8. An electrolysis cell in accordancewith claim 2, wherein the partitioning wall has a vertical extension andis further arranged such that an opening is provided below the lower endof the partitioning wall, and an opening of similar dimensions isprovided between the upper end of the partitioning wall and the upperlevel of the electrolyte.
 9. An electrowinning cell in accordance withclaim 1, wherein the gas separating chamber has a volume large enough toreduce electrolyte flow rates sufficiently to separate any gas containedin the electrolyte.
 10. An electrowinning cell in accordance with claim1, wherein one or more gas separating chambers can be arranged alongsideat least one side of the cell.
 11. An electrowinning cell in accordancewith claim 1, wherein the gas separating chamber is connected to atleast one gas exhaust system for extracting and collecting gases fromthe chamber.
 12. An electrowinning cell in accordance with claim 1,further comprising an exhaust system connected to an alumina feedingsystem in which hot off-gasses are used for heating alumina feed stockand/or used for scrubbing cleaning of off-gasses from the cell to removefluoride vapors, fluoride particulate and/or dust before entering a gascollection system.
 13. An electrowinning cell in accordance with claim1, wherein the auxiliary floor is provided with at least one holearranged below the cathode, whereby aluminium is allowed to pass throughsaid hole and to be collected in a metal compartment defined below saidfloor.
 14. An electrowinning cell in accordance with claim 13, whereinthe auxiliary floor material is selected from aluminium nitride, siliconcarbide, silicon nitride, oxide materials, refractory hard materialsbased on borides, carbides, nitrides, silicides or combinations orcomposites thereof.
 15. An electrowinning cell in accordance with claim13, wherein said aluminium in the metal compartment can be extractedfrom the cell via one or more surge pipes or siphons attached to thecell.
 16. An electrowinning cell in accordance with claim 1, wherein theanodes and the cathodes are of a monopolar type arranged in an alternatemanner, and further aligned vertically or inclined.
 17. Anelectrowinning cell in accordance with claim 1, wherein the anodes andcathodes are of the bipolar type aligned vertically or inclined.
 18. Anelectrowinning cell in accordance with claim 1, wherein the anodesand/or the cathodes consists of a plurality of smaller units integratedin one larger unit.
 19. An electrowinning cell in accordance with claim1, wherein the anodes are manufactured from dimensionally stablematerials, including oxide based cermets, metals, metal alloys, oxideceramics, and combinations or composites thereof.
 20. An electrowinningcell in accordance with claim 1, wherein the cathodes are manufacturedfrom electrically conductive refractory hard materials (RHM) based onborides, carbides, nitrides, silicides or mixtures thereof.
 21. Anelectrowinning cell in accordance with claim 1, wherein main surfaces ofthe at least one anode and the at least one cathode are arranged in amanner adjacent to a short side wall of the cell.
 22. An electrowinningcell in accordance with claim 1, wherein the cell has a lining thatincludes an electrically non-conductive material.
 23. An electrowinningcell in accordance with claim 22, wherein the material of the celllining is selected from aluminium oxide, aluminium nitride, siliconcarbide, silicon nitride, and combinations thereof or compositesthereof.
 24. An electrowinning cell in accordance with claim 22, whereinthe cell lining is manufactured from oxide materials.
 25. Anelectrowinning cell in accordance with claim 22, wherein at least partof the cell lining is manufactured from oxide or materials formed of acompound of one or several of the oxide components of the anodematerial, and additionally one or more oxide components.
 26. Anelectrowinning cell in accordance with claim 1, wherein the at least oneanode and the at least one cathode are connected to a periphery busbarsystem for electrical supply, wherein the connections can be introducedthrough the top, the sides or the bottom of the cell.
 27. Anelectrowinning cell in accordance with claim 1, wherein the anodesand/or cathodes connections are cooled to provide heat exchange and/orheat recovery from said anode/cathode, and/or temperature control. 28.An electrowinning cell in accordance with claim 1, wherein the anodeand/or cathode connections are cooled by means of water cooling or otherliquid coolants, by gas cooling or by the use of heat pipes.
 29. Anelectrowinning cell in accordance with claim 1, further comprising atleast one feeding tube for alumina where its inlet is located either ata position being close to a high-turbulence part in the electrolyte, andin the interpolar space between one anode and one cathode, or in the gasseparation chamber.
 30. An electrowinning cell in accordance with claim1, wherein the electrolyte flow pattern can be enhanced by the at leastone diaphragm, interior wall or skirt which is operable to deflect theupward flowing electrolyte into the gas separation chamber.
 31. Anelectrowinning cell in accordance with claim 1, wherein the diaphragm ismanufactured from aluminum oxide, aluminium nitride, silicon carbide,silicon nitride or combinations or composites thereof.
 32. Anelectrowinning cell in accordance with claim 1, wherein the diaphragm ismanufactured from oxide materials.
 33. An electrowinning cell inaccordance with claim 1, wherein the diaphragm is manufactured fromoxide or materials formed of a compound of one or several of the oxidecomponents of the anode material, and additionally one or more oxidecomponents.
 34. An electrowinning cell in accordance with claim 1,wherein the electrolyte comprises a mixture of sodium fluoride andaluminium fluoride, with additional metal fluorides of the group 1 and 2elements in the periodic table according to the IUPAC system, and thecomponents based on alkali or alkaline earth halides up to afluoride/halide molar ratio of 2.5, and where the NaF/AlF₃ molar ratiois in the range 1 to
 3. 35. An electrowinning cell in accordance withclaim 30, wherein the diaphragm is manufactured from aluminum oxide,aluminium nitride, silicon carbide, silicon nitride or combinations orcomposites thereof.
 36. An electrowinning cell in accordance with claim30, wherein the diaphragm is manufactured from oxide materials.
 37. Anelectrowinning cell in accordance with claim 30, wherein the diaphragmis manufactured from oxide or materials including a compound of one orseveral of the oxide components of the anode material, and additionallyone or more oxide components.